Indications for a Direct Singlet Fission Mechanism in TIPS-Pentacene Crystals from Hybrid DFT/MRCI and Molecular Mechanics Studies

A hybrid quantum mechanics/molecular mechanics setup was used to model the singlet fission (SF) of electronically excited 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) in the crystalline phase. The optically bright S₁^{<i>bright</i>} state possesses nearly identical excitation energies and oscillator strengths in the two nonequivalent dimer units with <b>large</b> and <b>small</b> structural overlap, respectively. A shearing/tilting motion of the two slip-stacked TIPS-pentacene building blocks is the key for stabilizing the singlet-coupled triplet-pair state, ¹(TT), in the <b>large/small</b> overlap situation. In both dimer models, the S₁^{<i>bright</i>} and ¹(TT) states swap energetic order upon geometry relaxation, indicative of strong nonadiabatic coupling between these states and a direct SF mechanism in TIPS-pentacene crystals, at variance with unsubstituted pentacene where charge-transfer contributions are substantial in V-shaped substructures. The overall energy balance <i>E</i>(S₁^{<i>bright</i>}) - (<i>E</i>(T₁) + <i>E</i>(T₁)) remains positive at all investigated nuclear arrangements. The ferromagnetically coupled ⁵(TT) state lies energetically too high to take part in the SF mechanism.